As one of the key individuals who brought the United States and North Korea closer than ever before to a deal on Pyongyang's nuclear weapons, former U.S. defense chief William Perry has a lot to say about dealing with the regime of Kim Jong Un and a planned landmark summit with President Donald Trump — and what it might mean for Japan.

Perry, who spoke at a forum in Tokyo on Wednesday, voiced cautious optimism about what would be the first-ever talks between a sitting U.S. president and a North Korean leader, but warned that the Trump administration would "do well to learn from the successes and failures of the past" and remember to work closely with American allies as it gears up for the summit.

His comments came the same day Foreign Minister Taro Kono met his South Korean counterpart, Kang Kyung-wha, in Seoul, where Kono stressed the need to "closely coordinate to realize the denuclearization of North Korea."